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How do comments work?

Comments exist so that users can talk about questions and answers without posting new answers that do not actually answer their parent questions. Comments are often used to ask for clarification on, suggest corrections to and provide meta-information about posts.

Comments are intentionally short, having maximum length of 600 characters, and allow only limited markup. URLs in comments automatically become hyperlinks. Each user may post only one comment every 15 seconds.

Comments are disposable: unlike posts, there's no revision history, and they can be deleted without warning by their authors, by moderators, and in response to flags.

Who can post comments?

All users may leave comments on their own posts and any answers given to their own questions. Users with at least 50 reputation may comment on any post. (There is no reputation requirement to post comments on MSO only; doing so elsewhere, including on other meta sites, requires 50 reputation.)

Who can edit comments?

Users may edit their own comments any number of times for five minutes after they are first posted. Edited comments are marked with a little pencil icon, and a mouseover tooltip over that icon will provide a count of how many times the comment was edited, like this:

Moderators can edit any comment at any time. Moderator comment edits are logged and visible to other moderators, but no revision history is kept. Such edits will also show the pencil icon.

In all other situations, comments cannot be edited. However, in lieu of editing, they may be deleted and resubmitted.

How can I format and link in comments?

Comments can be formatted with a subset of Markdown: bold (**bold**), italic (_italic_ or *italic*), bold italic (***bold italic*** or ___bold italic___) and code (`code`) are allowed. It is not generally possible to insert line breaks in comments, though some hacky workarounds exist.

URLs will be automatically converted into actual links. Links are also allowed by using the Markdown syntax [link text](http://myurl), and there are certain "magic links" that will be converted to real links automatically.

Spaces are not allowed in the comments URLs (even markdown), for example this: [URL](http://google.com/?q=stack overflow) will not generate a link. It needs to be encoded like this:
[URL](http://google.com/?q=stack%20overflow).

Who can delete comments?

A user may delete one of his own comments by clicking on the (x) icon that appears to the right of the comment's timestamp when the comment is moused over. A user can delete only one comment every five seconds.

Comments that are flagged by multiple users are deleted automatically. The number of flags needed is based on the comment's score. It currently takes "3 + (Score / 3)" flags to delete a comment.

Moderators can delete any comment, or purge all comments from a post.

When should comments be deleted?

Comments are temporary "Post-It" notes left on a question or answer. You should not expect them to be around forever: Once a clarification has been made, an edit added to the post to include new information, or the issue in the comment is otherwise resolved, it is subject to deletion. In reality, many obsolete or chatty comments remain untouched due to the high volume of comments posted, but this does not mean that they can't or shouldn't be deleted in the future.

What are automatic comments?

In several cases, the system will automatically add comments to posts on your behalf. You still own the comment, and it will appear the same as if you had manually posted the comment. You may delete the comment if you desire, but this is discouraged as it is helpful to the original poster: the automatic comments provide information on how to improve the post or why it is not appropriate.

When someone flags or votes to close a question as an exact duplicate, the system automatically posts a comment to that question which reads "possible duplicate of -link-", as long as no such link has already been posted in a comment. The comment will be automatically deleted if the question is closed as a duplicate.

When voting to close a question, the user can choose Off Topic and choose "Other", adding a custom reason. When doing that, automatic comment will be added on his behalf with the exact text he/she wrote, usually something like "This question appears to be off-topic because it is about [custom text here]". That user can delete or edit the comment within 5 minutes as any comment he posted, and that kind of automatic comment is not deleted when the question is put on hold. Note that this option is not present when flagging.

When choosing the "Recommend Deletion" action in the Very Low Quality review queue, an automatic comment will be added to the actioned post, unless the user selects the "no comment necessary" option. If the selected automatic comment is already present on the post, the existing comment will receive an upvote and a duplicate comment will not be added.

@balpha, you wrote bold and italic together is now working. But that's only true when using an asterisk for italic, but NOT when using the underscore: *underscore asterisk*, _asterisk underscore_, three asterisks, underscore, asterisk, two asterisks.
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ArjanFeb 25 '11 at 12:28

Oh wow, @balpha, you're right, that's actually how it's explained at meta.stackoverflow.com/editing-help as well! (I boldly combined the help shown in "How to format" in the right column when editing, which only says _italic_ or **bold**. But: indeed in "normal" posts the combination doesn't work either.)
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ArjanFeb 25 '11 at 12:40

@Arjan -- There are some quirks about Markdown that should be noted. Putting an emphasis character ( * or _ ) next to a character that is not on the whitelist will result in something that probably will not display the way it was intended to. I am not sure where this whitelist is published at, but I do know that square brackets ( [ , ] ) and double quotes ( " ) are not on it.
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Jay ElstonJun 16 '11 at 22:18

There have been some questions about comments that don't show up in the inbox, can you all clarify the bit about comment notifications? Do all comments leave an inbox notification?
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GabeJan 9 '12 at 15:04

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Comment are often more appropriate way to contribute when you don't have a definitive answer (but you want help to investigate or check some way). Personally I think each user should have the right do add both an answer and a comment. I don't want be offensive but the 50 reputation to gain comments right seems to me ridicolous.
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Eddie C.Sep 11 '12 at 8:48

I was having trouble leaving comments, and then it hit me . . . I need to be logged in. Hopefully this helps someone else who may be stumped at one of the more obvious solutions. :)
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CrazycoolcamSep 12 '12 at 11:26

"When should comments be used?" is not answered.
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gerritDec 18 '12 at 19:14

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I understand that comments need 50 reputation points to guard against spam, but it prevents a lot of discussion too... could it be done that comments are allowed to everyone, and those with rep < 50 be hidden by default, and shown after moderation? Something similar to this system will greatly help... esp for new comers requiring discussion/clarifications on specific questions. Or am I missing an already existing system in place?
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SiddharthaJan 2 '13 at 11:18

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What should we do if we don't have enough reputation to comment? Send a private message?? Create a new question??
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RezaRobJan 8 '13 at 4:39

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I agree with Siddhartha. Often times I find myself wanting to ask clarification on a question before answering or add to a discussion going on in comments but am unable to do so. 50 Rep is a bit ridiculous, doesn't anyone else think so? I've been around SO for over 3 years, even have the commentators badge but can't post comments any more (used to be able to before the rep limit). I consider my silence a sign of being able to find my own answers before spamming with easily answered stuff that plagues SO (which is encouraged by this limit). Couldn't, say, 25 be enough rep?
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AssimilaterJun 7 '13 at 20:45

There is recent discussion on Security.SE that I feel strongly about, regarding when someone prefaces a comment with a +1 or -1 and the comment. A +1 is the opposite of a -1, where the downvoter explains the criticism. If we encourage downvoters to explain their reasoning, then the same logic should be applied to upvoters as they praise.
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LamonteCristoAug 23 '14 at 13:30

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@RezaRob Move on to a different question that needs no clarification and answer it instead. Use the reputation earned from 5 upvotes on your answer to the second question to request clarification on the first.
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tepplesOct 26 '14 at 20:09